The authority of a U.S. president to singlehandedly order a nuclear strike has gone unquestioned by Congress for decades. Today, that changed. The Republican-led Senate foreign relations committee posed a key question. Could anything keep President Trump from launching a nuclear attack on his own? NPR's David Welna has the story.

DAVID WELNA, BYLINE: Democrats on the foreign relations panel pushed its Republican chairman, Bob Corker, to hold today's hearing. Ed Markey of Massachusetts led that effort.

(SOUNDBITE OF ARCHIVED RECORDING)

ED MARKEY: Thank you for having this very important hearing. I requested this several weeks ago, and I just think it's so important.

WELNA: Markey is the lead sponsor of a bill that would require congressional approval prior to any nuclear first strike. Connecticut Democrat Chris Murphy reminded his colleagues that this is hardly an abstract debate.

(SOUNDBITE OF ARCHIVED RECORDING)

CHRIS MURPHY: We are concerned that the president of the United States is so unstable, is so volatile, has a decision-making process that is so quixotic that he might order a nuclear weapons strike that is wildly out of step with U.S. national security interests.

WELNA: One of the witnesses was Brian McKeon, a nuclear arms expert at the Pentagon during the Obama administration. He reminded the senators that Congress alone under Article 1 of the Constitution has the power to declare war.

(SOUNDBITE OF ARCHIVED RECORDING)

BRIAN MCKEON: To be sure, the president possesses the constitutional authority to defend against sudden attack or to pre-empt an imminent attack. But Article 2 does not give him carte blanche to take the country to war.

WELNA: But General Robert Kehler, who commanded U.S. strategic forces under Obama, said the decision to use nuclear weapons was not up to Congress.

(SOUNDBITE OF ARCHIVED RECORDING)

ROBERT KEHLER: Only the president of the United States can order the employment of U.S. nuclear weapons.

WELNA: Still, Kehler said the military was not obliged to follow an illegal order, even from the commander in chief. That prompted this question from Maryland Democrat Ben Cardin.

(SOUNDBITE OF ARCHIVED RECORDING)

BEN CARDIN: Even if ordered by the president of the United States to use a nuclear first strike, you believe that under - because of legalities, you retain that decision to disobey the commander in chief?

KEHLER: Yes.

WELNA: Wisconsin Republican Ron Johnson pressed the retired general further on whether he would have disobeyed a nuclear strike order if he thought it was illegal.

(SOUNDBITE OF ARCHIVED RECORDING)

KEHLER: I would have said, I have a question about this. And I would have said, I'm not ready to proceed.

RON JOHNSON: And then what happens?

KEHLER: Well, you know, as I say, I don't know.

WELNA: The result, all parties agreed, would be a constitutional crisis. Idaho Republican Jim Risch warned his colleagues that this discussion could be sending North Korea the wrong signals.

(SOUNDBITE OF ARCHIVED RECORDING)

JIM RISCH: Pyongyang needs to understand that they're dealing with a person who's commander in chief right now, who is very focused on defending this country. And he will do what is necessary to defend this country.

WELNA: As the two-hour hearing ended, Massachusetts' Markey thanked Chairman Corker again for kicking off a much-needed debate.

(SOUNDBITE OF ARCHIVED RECORDING)

MARKEY: But I don't think that the assurances that I've received today will be satisfying to the American people. I think they can still realize that Donald Trump can launch nuclear codes just as easily as he can use his Twitter account.